The invention relates generally to optics, and relates more particularly to optical delay lines.
Optical delay lines are used a variety of applications including optical communications, radio-frequency (RF) photonics, future optical interconnects and the like. FIG. 1 is a cross-sectional view illustrating a waveguide structure 100 with tuning pads, such as may be incorporated in a conventional integrated optical delay line. The waveguide structure 100 comprises a substrate 102, a first oxide layer 104, a second oxide layer 106, a waveguide 108 and a heater 110. As illustrated, the waveguide 108 is disposed between the first oxide layer 104 and the second oxide layer 106, while the heater 110 is disposed on the second oxide layer 106, separated from the waveguide 108 by a distance, d, where d>approximately 1 μm.
The index of the waveguide 108 is tuned by manipulating the temperature of the waveguide core, using the heater 110. A common problem with waveguide structures like the waveguide structure 100, however, is that the heat produced by the heater 100 is not well confined. Only a small portion of the heat produced is actually utilized to tune the waveguide 108; hence, much of the heat consumed by the waveguide structure 100 (and associated delay line) is wasted. This negatively impacts the power consumption and operating speed of the delay line.
Moreover, many conventional optical delay lines are fabricated in fiber Bragg gratings (FBG) or in silica-on-silicon waveguide formats. In both cases (even the more compact silica waveguides), the footprint of the device tends to be quite large (e.g., approximately one to ten cm2 for a delay time of approximately two ns). In addition, the ability to accurately tune the delay, which is advantageous in most optical delay line applications, has not been satisfactorily addressed in existing optical delay lines.
Finally, material requirements (among other parameters) make integration of conventional optical delay lines with other passive and active photonic devices and electric circuits impractical.
Thus, there is a need for a method and an apparatus for tuning an integrated optical delay line.